Past research has found that a positive working alliance between clients and their case managers is modestly correlated with client outcomes. The current study tried to identify the predictors of the working alliance in a sample of 115 clients who were receiving services from Assertive Community Treatment (ACT) teams. All of the clients suffered from severe mental illness, had a substance use disorder and were homeless at baseline. Both the client's rating and the case manager's rating of the working alliance were assessed at 3 months and 15 months post baseline. Client characteristics, particularly motivation to change, explained more of the variance of the client's rating of the alliance than treatment variables or client change on the outcome variables. On the other hand, treatment variables (e.g., the amount of transportation services provided) and client change on the outcome variables explained more of the variance of the case manager's rating of the alliance.